This invention relates to a gas chromatograph with a flame photometric detector (FPD). More specifically, this invention relates to the gas chromatograph with the FPD which detects, in just one analysis, plural components included in a sample, each component having a different light-emitting wavelength.
Various types of detectors of the gas chromatograph are made practicable. The FPD, one of these detectors, is a detector for selectively detecting a sulfur compound and a phosphorus compound. Therefore, the FPD is used for various purposes, such as for an analysis of offensive odor elements like hydrogen sulfide, methyl mercaptun and the like, the detection of a very small quantity of sulfur included in chemicals, an analysis of residual pesticide, an analysis of biochemical elements, and the like.
FIG.3 is a schematic diagram of a detection cell of a typical FPD. A sample gas tube 10 connected to a column outlet of a gas chromatograph, a hydrogen supply tube 11, and an air (or oxygen) supply tube 12 are connected to a nozzle 13 having a gas spouting outlet 14 directed upward. A transparent crystal cylinder 15 is placed around the nozzle 13. The whole of the crystal cylinder 15 is covered with a heat insulation block 16 made of metal. A heater and temperature sensor, not shown in the Figure, are embedded in or attached to the heat insulation block 16. According to a known feedback control of the heater using temperature information, the heat insulation block 16 is kept at a designated temperature. A designated part of the heat insulation block 16 is bored to define a detection window 17. A wavelength filter 19 is placed through a cooling fin 18 outside the detection window 17. Further outside the wavelength filter 19, a photomultiplier tube 20 as a detector is placed. The wavelength filter 19 is designed to transmit only the light with the target wavelength corresponding to the target sample component.
An operation principle of the above mentioned FPD is as follows. Carrier gas such as nitrogen gas supplied through the sample gas tube 10, hydrogen gas supplied through the hydrogen supply tube 11, and air supplied through the air supply tube 12 are mixed at the tip of the nozzle 13, then a hydrogen flame 21 is formed by combustion of this mixture. For example, when a sample component flowing out of the gas chromatograph column is introduced into the flame, it is combusted and the light having the target wavelength corresponding to the sample component is emitted. Especially, a reducing flame of peroxide emits the light with wavelengths of 394 .mu.m and 526 .mu.m through combustion of a sulfur compound and a phosphorus compound, respectively. The light emitted from a combustion part 22 for the sample component in the hydrogen flame 21 passes the transparent crystal cylinder 15 and reaches the wavelength filter 19. Only the light with the target wavelength selectively passes through the wavelength filter 19 and reaches the photomultiplier tube 20. By these steps, component detection with very high selectivity becomes possible. The crystal cylinder 15 is used for protecting the wavelength filter 19 from tarnishing over from steam or soot and the like produced by the hydrogen flame 21.
As mentioned above, in the FPD, the light of the target wavelength which is different depending on material of the target component such as phosphorus, sulfur, and tin is emitted. In the FPD, shown in FIG.3, the wavelength filter 19 has to be changed depending on each different target component in order to detect the different target components. Therefore, in this single filter type of FPD, it is impossible to detect, in one analysis, both a sulfur compound and a phosphorus compound which are separated in a column to exit from the column at different times. On the other hand, there is another type of FPD which has two different wavelength filters transmitting the light of the target wavelengths corresponding to two components, respectively, and two photomultiplier tubes detecting the light transmitted by the wavelength filters, respectively. In this dual filters type of FPD, a phosphorus compound and sulfur compound, which are separated in the column to exit at different times, are combusted in a hydrogen flame. The light with each of the target wavelengths corresponding to phosphorous and sulfur compounds emitted from the flame is selectively transmitted through a respective one of each of the wavelength filters, and each transmitted light is detected by a respective one of each of photomultiplier tubes.
In the FPD, shown in FIG.3, when a mixture rate of hydrogen gas and air changes, the size of the hydrogen flame 21 changes and also the temperature distribution in the flame changes. Since each mechanism for emission of light in sulfur, phosphorus, and tin, is different from the others, each mixture rate of hydrogen gas and air for maximizing a light amount emitted by combustion is also different depending on each component combusted. Therefore, it is desirable to set a proper flow amount of hydrogen and air supplied to the nozzle 13 depending on the target component in order to obtain a large amount of light and to enhance the detection. In the single filter type of FPD, shown in FIG.3, the most suitable combustion can be realized by changing the wavelength filter into a proper wavelength filter for the target component and resetting proper flow amounts of hydrogen gas and air depending on the target component.
On the other hand, when plural components are measured in one analysis (one sample injection) with the dual filters type of FPD, each flow amount of hydrogen gas and air is set so that a mixture rate of them becomes intermediate among the optimum mixture rates of those target components. In this case, since the combustion of the components is not optimum and each amount of light emitted is not maximum, the detection for each of the target components is not good.